Modern dwelling units and other structures commonly incorporate some form of air-conditioning system. Use of air-conditioning systems in residential applications has become more and more commonplace over the years. Many other structures, such as factories and office buildings, integrate air conditioning systems into their facilities.
Most air-conditioning systems are structured according to traditional heat pump principles. In a typical cooling system, a refrigerant is used as a working fluid in a closed-loop heat pump application. Two types of systems have evolved in most regions of the country; integrated systems and split-systems. Integrated systems comprise a single operational unit that comprises all of the components necessary to pump heat. Split-systems segregate the functionality of the heat pump into two sections, one for heat removal and the other for heat dispersal.
Split-system air conditioning apparatus have found favor in small volume applications including single family dwelling units, apartments, small offices and other small industrial facilities. These split-systems typically comprise an indoor unit and an outdoor unit. In the air conditioning trade, the indoor unit is commonly called a “heat exchanger” because it exchanges cooler air for warmer found in a comfort volume. Heat from the comfort volume is carried away by the working fluid. The outdoor unit is normally referred to as a “heat pump” or a “compressor”. The outdoor unit typically comprises a compressor that is used to introduce work into the system effecting the heat transfer cycle.
The indoor unit typically comprises an evaporator and a fan element. The fan element is used to direct warm air from the living space, i.e. the comfort volume, through the evaporator. As the warm air from the comfort volume passes through the evaporator, the working fluid, i.e. the refrigerant, absorbs heat from the air. The air that leaves the evaporator is cooler than the air entering the evaporator. The net effect of removing heat from the circulating air reduces the temperature in the comfort volume.
As the working fluid traverses through the system, it typically enters the evaporator as a very cool liquid. As the working fluid absorbs heat from the warm air passing through the evaporator, it will generally experience a rise in temperature. This rise in temperature causes the working fluid to change state from a liquid to a vapor. The vaporized working fluid then leaves the evaporator and is directed to the outdoor unit.
As the vaporized working fluid enters the outdoor unit, i.e. the “heat pump”, it encounters a compressor. The compressor pressurizes the working fluid; which is in a vaporous state. In many cases, the working fluid will reach a super-heated state after compression.
The high-pressure and high-temperature vapor then enters a condenser. The outdoor unit typically further comprises a fan that drives outside ambient air through the condenser. As the working fluid traverses the condenser, it loses some of its heat to the outside air. As the working fluid leaves the condenser, it typically remains in a pressurized, vaporous state. The working fluid then passes through an expansion valve. This allows the pressure of the working fluid to be reduced. This pressure reduction results in condensation of the working fluid. After passing through the expansion valve, the working fluid becomes a cool, low-pressure liquid. The cool liquid working fluid is routed back to the indoor unit to complete the cooling cycle.
Most of these traditional air-conditioning systems utilize an electric motor to drive the compressor included in the outdoor unit. The work imparted by the electric motor onto the compressor requires significant energy. In many instances, the amount of work expended will significantly increase the cost of electric utility charges incurred by the occupant of the home or business facility.
Several alternative means of cooling an indoor space have been suggested in attempts to reduce or completely avoid electric power consumption. In one known method, a Sterling cycle has been used to create an air-conditioning system driven by waste heat captured from other systems such as a water heating apparatus disposed in the facility. When waste heat is not available, a Sterling cycle based cooling systems needs to burn some other fuel in order to maintain comfort in the target environment.
A Sterling cycle air-conditioning system may also be driven by solar energy. The notion of using solar energy to drive cooling systems is quite intriguing. This is especially true in light of the fact that air conditioning systems are typically used during hot summer months when solar incidence is high. One problem with these Sterling cycle apparatus is inefficiency. The Sterling cycle itself is not especially efficient. Hence, large solar arrays are required to obtain the power needed to cool even a moderate sized dwelling unit or office complex.
One other disadvantage with Sterling cycle systems is the fact that when radiant energy from the sun is not directly available, ancillary heat sources are required to maintain the cooling cycle. Many prior art Sterling cycle based systems rely on natural gas heating elements to augment the Sterling cycle when solar radiation is insufficient.
Solar energy has been used to drive a simple Rankine cycle based motor generator. In these prior art systems, inefficiency is again the compromising factor because the solar radiation captured through the Rankine cycle must be first converted into rotating work by some form of a turbine. The work produced by the turbine is then used to generate electricity. The electrical energy is then converted into rotating work by a motor that drives a compressor. The compressor is used to force a working fluid through a refrigeration cycle. Each of these conversion stages introduces significant inefficiencies in the final air condition system structure.
A Rankine cycle solar air conditioning system still needs to be augmented with utility power when solar energy is not sufficient to maintain comfort in the target cooling volume. This further complicates Rankine motor-generator systems because of the need to synchronize the AC output of the motor generator to the power line provided by the utility company.
Solar energy can be used to augment conventional, electrically driven air conditioning systems. One known technique uses photovoltaic cells (a.ka. solar cells) to generate DC power. Photovoltaic cells, though, are typically not very efficient and they are still very expensive. The surface area of a suitable solar collector needed to cool a typical residential unit may be too large and expensive to be practical. Even more discouraging is the fact that a solar cell has a limited life and the output produced by a solar cell drops off sharply with age.
Techniques relying on electrical energy created by photovoltaic cells must also include an inverter that is capable of converting DC power provided by the photovoltaic cells into an AC voltage that is synchronized to the utility line. This is not a simple process because the output of the inverter must be continuously adjusted in voltage, frequency and phase to ensure delivery of power into the utility power line. Typically the phase of the inverter's output must be continuously adjusted in phase relative to the phase of the utility power to ensure positive power flow.
Notwithstanding the inefficiencies associated with these prior art techniques, the need to augment any solar based air-conditioning system with utility power complicates the overall system design. The complicated structures necessary to combine solar derived AC or DC power with the AC power obtained from a utility company result in additional system costs that may prove prohibitive and commercially unviable in most applications.